Plants in Relation to Man 457 



section of the United States. Masses of rock and earth were 

 taken up by the glaciers and while being pushed along were 

 crushed into fragments of different size, and deposited as 

 moraine. The drift formed by glaciation differs greatly in 

 character and value. On Cape Cod the soil is sandy. Over 

 a large part of the eastern glacial region the soils are coarse, 

 a mixture of sand, gravel, and clay, with a liberal supply of 

 bowlders often curiously placed. In the western area, as in 

 Illinois, Iowa, and Nebraska, the texture of the soil is finer and 

 is exceedingly productive. 



There is a third kind of soil known as the lacustrian. This 

 is the sediment deposited on the bottoms of lakes that have 

 since disappeared. Of the many lake plains in North America 

 that of the rich wheat-growing valley of the Red River of the 

 North is the most interesting. This area, which is greater 

 in extent than the combined area of the Great Lakes, comprises 

 what are now large parts of Minnesota, the Dakotas, and Man- 

 itoba. While the glacier was melting and slowly receding 

 northward, it blocked the outlet of this river and caused the 

 formation of a lake called Lake Agassiz. When the river was 

 opened, it drained the lake. The land is very level, and the 

 soil is of the right texture and richness to produce wheat most 

 profitably. 



221. The soil's moisture. Soil must, however, be more 

 than a mixture of rock fragments and decayed organic matter. 

 You find that it is moist at different depths ; but the moisture 

 is not like free water (page 466). This is generally called soil 

 water or soil moisture, as it is an indispensable part of a pro- 

 ductive soil. Large areas of the United States do not receive 

 enough rainfall to produce more than a scanty vegetation. 

 Such soil is arid. Soils that receive and retain enough moisture 

 for plant production are humid. Plants that grow on arid 

 soil are either specially adapted to prolonged drought or pro- 

 vided with long roots that reach deeply into the earth to find 

 necessary moisture and food. 



